1. Field of the Invention
The present invention relates to a sheet material and a production method thereof, having a drying process for a sheet material in a wet condition such as paper or some types of wet process-nonwoven fabric, and an apparatus for drying sheet material.
2. Description of the Related Art
A typical method for drying a paper web on a paper-making machine generally involves transfer of heat from a dryer cylinder having a second-kind pressure vessel structure (Japanese Boiler and Pressure Vessel Code similar to ASME Code in USA, DIN Code in Germany, and ISO Standard) internally heated with medium to low pressure steam, or from a cylinder internally heated by direct combustion or electromagnetic induction heating. These cylinders are surrounded by an open hood having only a roof and a curtain-type wall, or a loosely sealed hood having an opening for many sheets and ropes, a drive shaft and a piping, a duct, and a door moving up and down, having many gaps for moving up and down.
A large volume of medium temperature, low humidity air heated by low pressure steam is blown into respective parts of the hood, mainly into a pocket portion between dryer cylinders. The steam evaporated from the wet sheet material and the air blown mainly into the pocket portion are removed together with leaking air into the hood from many hood openings and gaps, under medium temperature and low humidity conditions with the relative humidity being 30% or less, so that dew condensation does not occur in the hood.
The absolute humidity of the discharge gas is low and the dew-point temperature is also low. Since heat transfer in condensation is quite little, the only way to heat the fresh is to heat by a sensible heat. The heated moist air including a large amount of discharged steam is directly released to the atmosphere. Hence an enormous amount of heat and water are wasted. Moreover, the air supply and discharge blower supplies and discharge large amount of air, thereby wasting a large amount of power.
The dryer cylinder having the second-kind pressure vessel structure consumes a large amount of low pressure steam of about 2 to 4 kg/cm2 via a rotary joint. The steam condensed inside is extracted and returned to a boiler water supply. During this time, the drain gradually decreases in internal pressure in the tube through each flush tank of the drainage system, and finally returns to the atmospheric pressure in the drain tank of the power station, and re-evaporates and radiates heat.
In Tappi Journal published in May 2000, there is proposed a method of drying a wet sheet material at a high temperature in excess of 100xc2x0 C., while restraining expansion and contraction thereof, wherein the sheet material is held between two endless steel belts, high pressure cases opened towards the belt surface are provided above and below thereof facing each other, to perform high pressure steam heating and high pressure water cooling, to thereby immediately condense the evaporated steam on the cooling surface, and the condensate is drained onto an endless fabric belt and carried away.
This method requires precious high pressure steam. However, since this involves high temperature steam heating in a closed vessel, the temperature in the vessel is restricted by the saturated temperature of the steam. With the impingement drying method of the present invention, the sheet material can be heated using super-heated steam of 250xc2x0 C. substantially at atmospheric pressure (needless to say, a direct combustion method is desired for the heating method using a heat exchanger). In the closed vessel, the steam temperature of 250xc2x0 C. means the steam at a pressure of 40.6 kg/cm2. Therefore, steam turbine power generation using the steam pressure difference, which is normally performed in the central power station is not possible. Moreover, this method requires a large quantity of cooling water, which can only be recycled as low temperature warm water.
Furthermore, the above method requires two pressure vessels facing each other and having a strong frame structure mechanically corresponding thereto, enormous investment in plant and equipment involving high pressure heat exchangers and high pressure pumps, and power and steam expenses. It has of course a large effect in improving some aspects of quality, such as paper strength, but on the other hand, the density becomes high due to the high compressive force by the upper and lower steel belts, thereby making the thickness of paper very thin.
The heating surface is smooth like a Yankee glazed surface, but on the other hand, the cooling surface is a corrugated surface with a fabric belt mark, and hence, there is a large difference between the front surface and the back surface. Therefore, there are problems in that it is not suitable except for liner board for parts of cold storage, and there is no heat saving effect.
For drying of thin sheet materials, such as thin paper and toilet and tissue papers, high temperature gas of 350xc2x0 C. by natural gas or kerosene burner using fresh air or a part recycled moist air are impinging over the exposed sheet material without dryer fabric belt, under a high speed of 70-120 m/sec. from the outer surface of a single large-diameter dryer cylinder (generally called a Yankee dryer, with the sides and the entrance and exit for the sheet material thereof completely opened in the interior space) is heated by medium pressure steam of about 10 kg/cm2, and installed so as to open to the outside, which has a canopy hood disposed in a central top half portion of the cylinder. The sheet material has a Yankee glazed surface on one side only, and the other side remains rough. The use of manufactured product is therefore limited somewhat to such uses as a wrapping paper with one glazed side, tissue paper, and crepe paper. Moreover, there are problems related to fire accident by high temperature combustion gas and a diameter of the dryer cylinder becomes a problem with respect to transportation thereof and the diameter cannot be further larger.
Actually, for the paper industry that essentially requires a large amount of pure river water, it is common worldwide for the consolidated area of the paper mills to be in an area blocked by steep ridges, with narrow roads. Therefore, the width of the paper-making machine is restricted by the dryer width capable of truck transportation.
Moreover, there has also been proposed a method of recycling the evaporated saturated steam as a part of the heat supplied to drier cylinders, which are pressurized vessels, without supplying air to the loosely sealed hood. However, in practice, it is difficult to eliminate air completely from the loosely sealed hood of a large volume having covered passageways on the opposite sides thereof.
Furthermore, volumes of air enter into the sealed hood together with the sheet material and the endless fabric belt supplied continuously through an opening from the wet part. When the sheet material breaks due to internal shrinkage and forcible driving, and the low temperature wet paper is absorbed to the dryer cylinder and breaks over and over, it is necessary to stop the machine, open the loosely sealed hood, and after the interior of the hood has been cleared of the breakage, close the hood and re-start. Whenever this happens, the inside of the hood is replaced with the air. Under these circumstances, it is impossible to keep air entering into the hood below 4% as is generally recommended (oxygen: 0.84%, steam partial pressure: 729.6 mmHg) with the related art.
Further, the saturated steam inside the sealed hood enters into the wet zone, when cooled by the wet sheet material as well as external air entering the sealed hood from many openings. Then, moisture condensed on the metal surfaces of the hood and dryer frames may drip onto the sheet material, creating staining defects. Because of such inherent problems, this proposed technique has not been commercialized yet.
As is described in the introduction of xe2x80x9cTheory of Dryingxe2x80x9d, Article 6.2 in Pulp and Paper Manufacturing Technology, Volume 6, xe2x80x9cPaper Makingxe2x80x9d, edited by JAPAN TAPPI, published in December 1998, stating xe2x80x9cThe form most popular at present in drying methods in paper-making machines is a form in which dryer cylinders are heated by pressurized steam therethrough, and wet paper is pressed against this heated surface, . . . xe2x80x9d, it has been a rigid rule to dry ordinary paper by dryer cylinders whose inside was heated by pressurized steam, with the exception of the case where the inside thereof is heated by direct combustion, in part of home paper mill in old days. The current dryer cylinder has a second-kind pressure vessel code (in Japan) structure, and an internal heating structure mainly consists of conduction heat transfer from a low temperature cylinder in which heating is performed from the inside with low pressure steam.
Since it has been heretofore believed even by those scientists in this field that the drying rate is increased by a difference in the steam partial pressure in the air, it has been considered to be advantageous that the evaporation surface of the sheet is heated to a high temperature to create high steam partial pressure, and dried air having low steam partial pressure (having low absolute humidity) is used as the heated air.
Generally, the drying of a wet paper web (sheet material) is generally based on letting both sides of the wet paper web alternately come into contact with the dryer cylinders heated by low pressure steam so as not to loose smoothness due to curling and cockling. Furthermore, dimensional stability is provided by sandwiching the wet paper web between the dryer cylinder and an endless fabric belt so as to restrict free shrinkage in the cross direction of the paper. However, the drying of paper is mainly performed in a free traveling section between drying cylinders, and the cross direction of the paper greatly shrinks in the free running section, such effort has had minimal effect.
Furthermore, with increasing production speed of paper making, the number of dryer cylinders has also been increased from a dozens to nearly one hundred cylinders, so that the constitution of the apparatus becomes complicated. Therefore, in the drying section, complicated control is performed by a individual electric drive by pulling the paper with a uniform tension to cope with shrinkage in the cross direction and the travelling direction. Furthermore, the fabric belt and the fabric belt suction roll as well as an air boxes are employed to prevent paper break and achieve uniform drying in both longitudinal and transverse directions. Nevertheless, breakage of paper does occur frequently between the dryers or the dryer sections, and when the paper web is broken, the paper machine must be stopped. In this case, the loosely sealed hood must be opened to remove the breakage before the machine can be re-started. The existing process therefore requires much time and manpower, causing a drop of productivity, and maintenance problems can present problems to personal safety in some cases.
The multi-cylinder type dryer cylinders have a diameter reaching up to 2 m from 1.2 m to 1.5 m, and the maximum cylinder width has also been increased to a size in excess of 10 m. Therefore, extra attention must be paid to the transport problem from the cylinder foundry to the mechanical assembly plant and to the final paper mill. The dryer cylinders made of cast steel as the second-kind pressure vessel having a steam pressure as high as 2 to 4 kg/cm2 have a possibility of explosion due to casting nest or deterioration with lapse of time, which presents a problem of accidents resulting in injury or death. Moreover, in the casting industry, location of plants in Japan becomes more and more difficult due to their very bad working environment and dust pollution. Therefore, the dryer cylinders made by casting have to rely on imported products from overseas, and this is becoming a vulnerable point in the paper industry in Japan, including shipping costs.
Moreover, with an increase in the speed of the drying section, for example, 1800 m/min. in the case of newsprint, there is a case where the drain condensed inside moves around in the internal circumference of the dryer cylinder, causing problems in that draining of the condensates does not take place smoothly, causing uneven drain film it thickness across the cylinder width.
Another serious problem is associated with a huge volume of steam required for the dryer section From 1.5 to 3 tons of steam is required for every ton of dried paper produced (depending on the raw material or paper grade). Hence the paper industry becomes an industry consuming lots of energy.
As the loosely sealed hood, a hood has been developed in recent years by improving the insulation performance of the hood so as to obtain a dew point of around 60xc2x0 C. (though differing in each section of the hood, in an actual example, as the average air condition at the exit of the discharge fan, wet-bulb temperature: 63xc2x0 C. at a dry-bulb temperature of 110xc2x0 C., dew-point temperature: 60.5xc2x0 C., absolute humidity: 0.1553 kg steam/kg dry air, steam partial pressure 151.8 mmHg, relative humidity: 14.1%). However, the oxygen concentration is still 16.8%, and the risk of fire or dust explosion due to broken paper or paper dust has not yet been solved. The volume of air required has also been lowered significantly, but most of the important steam vapor evaporated is still discharged to the atmosphere, and a problem remains of generation of enormous amounts of white smoke (produced by condensation of moisture in the discharged moist air), particularly during winter and early spring seasons. In some locations, this presents a serious hazard to residents and traffic.
In the Kyoto convention for arresting global warming held in Dec. 11th, 1997, the Kyoto Protocol having legal obligation was adopted for the first time in the world, requiring that greenhouse effect gas such as carbon dioxide (CO2) is to be reduced by 5.2% (6% in Japan) compared with 1990 over five years before and after year 2010. In addition, an international joint research group including the National Environment Laboratory in Japan published in May 8th, 2001 that steam content in the air was increasing in the last 50 years, and human activity was one reason for that. The steam content in the atmosphere has a greenhouse effect exceeding that of carbon dioxide, and an increase in concentration in the stratosphere has an action of destroying the ozonosphere due to freon or the like. However, a comprehensive study related to concentration fluctuation has been performed for the first time, and it is expected that the conventional global warming research will be reconsidered. It has been found that the steam concentration in the stratosphere is about 4 to 6 ppm, which has increased by about 2 ppm in about 45 years from the mid 1950s. About half of the increase of steam is generated by methane gas, whose concentration is increasing in the air, which is oxidized in the upper atmosphere and turned into water. The cause of the remaining half is uncertain. Japanese pulp and paper industry got over the thermal energy crisis, which caused a sharp rise in the oil price due to two oil shocks in 1973 and 1979, after pathetic efforts, involving steps from the chemical recovery of spent cooking liquor, the chip cooking, and pulp washing, up to stock preparation and paper making. As a result, the energy-saving level is at the top of the world, and further energy saving of 6% reduction over the level of 11 years ago will be nearly impossible. We thus have no alternative but to proceed with afforestation in developing countries to increase the CO2 absorbing effect of the forests.
Moreover, so long as the moist air is used as the transfer gas for the vaporized steam in the loosely sealed hood, the upper limit of dew point temperature is around 60xc2x0 C. When the quantity of dry air is less is compared with the quantity of the evaporated steam, saturation of the air can occur easily, and air is easily condensed in the sealed hood. In this case, the surface of the paper is contaminated due to drip of condensation and papermaking becomes difficult. Furthermore, the temperature of the sheet material on the side in contact with the dryer cylinder reaches about near 100xc2x0 C., but the temperature thereof on the contact side with fabric belt is restricted to about 90xc2x0 C., due to the latent heat loss of the removed moisture from the paper cylinder contact surface.
Furthermore, voids in the fabric belt are filled with vaporized steam and condensed moisture, and there is a temperature gradient between the outside layer and the inside layer, the former is in equilibrium with the dew-point temperature of the moist air (65 to 70xc2x0 C.) and the latter is in contact with the sheet material (about 85xc2x0 C.), thus water evaporation from the sheet material is greatly restricted. For this reason, there is little drying of the sheet material taking place in the zone of the dryer where the sheet material is in contact with the fabric belt, and most of the drying actually takes place in the free running zone between the plurality of dryer cylinders, where the moisture is evaporated directly from the surface of the sheet material.
From this reason, an effort has been made to improve the dimensional stability of the sheet material by holding the sheet between the dryer cylinders and the endless fabric belt for drying the sheet and restricting free shrinkage of the sheet material. However, only about 20% of drying takes place in the fabric belt-restrained zone, and about 80% of the moisture is evaporated in the free running zone of the sheet material where free shrinkage is possible.
Recently, an attempt has been made to solve the above described problems by arranging multi-cylinder type dryer cylinders in a single row and providing a large-diameter suction fabric belt roll close to the dryer cylinders as much as possible, to thereby reduce the free running zone between the adjacent dryer cylinders. However, there is a large difference between the thickness of the endless fabric belt and the thickness of the sheet material, and these reverse the inside and outside alternately, and hence the radius differs largely between the dryer cylinder and the suction fabric belt roll portion. Hence, a difference in the surface velocity appears, frequently causing a problem in that the wet sheet material is broken.
Initially, the arrangement of the suction fabric belt roll was changed to give an upward directed arrangement and a downward directed arrangement for each section, but there is a problem in paper delivery at the time of paper break, and only the downward directed arrangement could be adopted. In this case, a large problem occurred in that drying only from one side of the dryer cylinder caused dry curling of the paper. As a measure against this problem, an attempt has been recently made in which air caps are provided in key points of the downward directed single-row dryer cylinders in the terminal stage of the drying part, to blow heated air of about 150xc2x0 C. at a rate of 100 m/sec., which is similar to the heated air impingement drying method registered by the present applicant. However, this has an assisted drying capacity correcting only curling, and uses low-pressure steam for the circulated heating source of the blown heated air. As a result, it has to be used in a range of low dew-point temperature which does not infringe the registered patent of the present applicant (Japanese Patent No. 3007542, U.S. Pat. No. 5,553,392, U.S. Pat. No. 5,647,141, EC Pat. Registration decided). Therefore, this has an enormous heating requirement as described later in detail, and has to be limited for correcting curling.
It is an object of the present invention to provide a sheet material and a production method therefor, and an apparatus for drying sheet material, wherein the above described many technical problems are overcome, high-speed drying several times higher than before is achieved by means of impingement energy, without causing breakage of the paper, thereby succeeding in the reduction of heating requirements to about one eighth, as well as increasing physical strength by decreasing a glass-transition temperature of the paper, and the paper is made porous by instantaneous evaporation to decrease the bulk density thereof by 31%.
In order to solve the above described problems, the sheet material and the production method thereof is characterized in a process for drying a sheet material in a wet condition, wherein the inner periphery of the sheet material is supported by a rotor having a rotatable arc-shaped smooth surface, within a sealed hood interior set to a predetermined pressure and temperature, the outer periphery thereof is clamped by an endless heat-resistant belt movable in synchronous with the rotation of the rotor and having gas permeability of at least 7,500 cm3/cm2/min., under a tension capable of restraining dry shrinkage of the sheet material applied by means of a belt roll, and heated gas is blown from the outer peripheral direction of the rotor towards the belt in a drying zone selected according to the intended use, to thereby dry the sheet material mainly by external heating.
The apparatus for drying sheet material of the present invention is characterized by a drying apparatus for a wet sheet material, wherein there are provided, in optional sections according to intended use, a plurality of rotors having a smooth outer peripheral surface, for propelling the sheet material on the convex curved surface, while supporting an inner peripheral surface of the sheet material; an endless gas-permeable heat-resistant belt coming in contact with an outer peripheral surface of the sheet material and movable in synchronous with the rotation of the rotor, while holding the sheet material between the rotor and the belt; a multiplicity of belt rolls for applying a tension capable of restraining dry shrinkage of the sheet material to the heat-resistant belt; and a heated gas supply section for blowing heated gas from an outer peripheral direction of the rotor towards the heat-resistant belt, and the rotor is heated from the outer peripheral direction by the heated gas having passed through the belt-like body, which is supplied from the heated gas supply section.
The present inventor has found in the process of development, that excluding the passing section from of the entry and exit portions for the hood where the sheet material passes through, if a rotatable rotor internally heated by low pressure steam is impingement-heated from the outer periphery of the rotor, by heated gas (super-heated steam, heated moist air having a dew-point temperature of at least 65xc2x0 C., nitrogen gas containing a small amount of solvent or steam) higher than the saturation temperature corresponding to the supplied steam pressure, with the rotor including the sides thereof being completely sealed by a heat insulating material, then the surface temperature of the rotor is heated up to about 25xc2x0 C. less than the heated gas temperature, and the low pressure steam supplied as the heat source for the rotor is not condensed, but is overheated and discharged as super-heated steam, and hence the drying rate can be increased rather than by stopping the low pressure steam supplied to the rotor.
Accordingly, a sheet material and the production method thereof, and a drying apparatus for the sheet material can be provided by blowing the heated gas from the outer peripheral direction of the rotor supporting the sheet material towards the belt, as in the present invention, wherein high-speed drying several times higher than before can be achieved by the impingement energy mainly by external heating. As a result, the required heat can be reduced to about one eighth, the physical strength of paper can be significantly improved by a fall of the glass-transition temperature of paper, and the paper is made porous by instantaneous evaporation to decrease the bulk density by 31%. Moreover, by blowing the heated gas while holding the sheet material between the gas-permeable belt and the rotor to thereby dry the sheet material while restricting the expansion and the contraction thereof, dimensional stability such as extensibility in water can be increased. Also, by selecting the front or back surface of the paper to adjust the drying conditions, curling degree in the CD and MD directions can be adjusted to prevent the occurrence of curling beforehand. Furthermore, by propelling the sheet material, while holding it between the gas-permeable belt and the rotor, breakage of paper can be eliminated. Meanwhile, microorganisms can be sterilized substantially by 100%, by directly heating the moisture in the sheet to at least 100xc2x0 C. in the wet zone by heating with high-temperature super-heated steam, and by the water evaporation from the inside of the sheet by means of a pressure flow. As a result, there is a possibility to produce sterilized paper for food and medical applications, such as cup base paper and milk carton base paper. In xe2x80x9cNew Technology in Food Processingxe2x80x9d published by CMC Co., Sumio Kawai, it is described in xe2x80x9cDrying and sterilizing effects of bread crumbs in super-heated steam fluidized bedxe2x80x9d that in an atmosphere of super-heated steam of 150xc2x0 C., the viable count of about 700 millions was reduced such that the remaining viable count was only one after one minute later. On the other hand, in heated air of 150xc2x0 C., the remaining viable count was still 36,000 after five minutes later. In the drying of wet strength paper or dry strength paper, the moisture in the sheet is directly heated to at least 100xc2x0 C. in the wet zone by heating with high-temperature super-heated steam, and by the evaporating the moisture from the inside of the sheet by means of pressure flow, the super-heated steam in excess of 100xc2x0 C. impinges upon the wet strength agent or the dry strength agent, to advance the paper strength manifestation mechanism in the steam atmosphere. As a result, it becomes possible to manifest the paper strength even if the aging time normally required after paper finishing.
In development tests for a new drying method extending over a long period of time, since heat-resistant lubricating oil was expensive, at first an attempt was made to expose the bearing of the rotor to the air by disposing the sides of the rotor outside of the hood and seal the outer peripheral surface and the side portions of the end plate. However, complete sealing was difficult, and the air easily entered. As a result, it was found that it was necessary to maintain the internal pressure of the hood relatively high, such that the leaking steam is condensed on the sealed surface to thereby effect sealing, in order to maintain the 100% super-heated steam atmosphere.
Moreover, heat loss from the cylinder sides was also large, and as described above, it was confirmed that internal heating in the cylinder should not be stopped. Thereafter, the entire surface of each cylinder was tightly closed, and remodeled with a brush seal method. As a result, the leaked amount of steam has decreased sharply, so that not only high dew-point moist air but also a 100% super-heated steam can be sufficiently ensured.
In tests after completion, when the wet sheet material is held between the rotating cylinders and the endless gas-permeable belt tensioned with a high tension having a width of 1.5 kg/cm, preferably, at least 2 kg/cm, and quickly dried (dried by impingement) in a super-heated gas atmosphere of at least 130xc2x0 C., it was confirmed that the occurrence of breakage in the travelling direction resulting from shrinkage within the fiber caused by interfiber bonding or shrinkage in the cross direction can be restrained by nearly 100%. Thus, according to the present invention, it becomes possible to produce sheets having high dimensional stability and a small aspect ratio, without causing dry curling.
The softening point temperature of lignin and hemicellulose, drops in an atmosphere of super-heated steam (steam 100%) and in an atmosphere of heated moist air having high absolute humidity (at least 1 kg/kgxe2x80x2 DA), respectively (although the softening point of dried lignin is 134 to 250xc2x0 C., the water saturated lignin falls to 72xc2x0 C.). The above phenomenon is caused by a drop of the glass-transition temperature. Therefore, flexibility of fibers is increased, and wet sheet strength is substantially increased due to the covalent bonding (ether bond, ester bond) of the hydroxyl group in cellulose with other substances contained in the wood. By impingement drying with super-heated steam in excess of 150xc2x0 C. (or high temperature, high humidity heated moist air having a dry-bulb temperature of at least 150xc2x0 C. and a dew-point temperature of at least 65xc2x0 C.) to thereby decrease the softening temperature, physical properties such as DRY tensile strength, WET tensile strength, water immersion elongation and bursting strength can be considerably improved, compared to the conventional drying by dryer cylinders in which the inside thereof is heated by steam. It has been found that of the various physical properties of paper described above, the improvement in strength of various kinds of paper is notable not only in the wet paper mainly composed of UKP (Unbleached Kraft Pulp) and BKP (Bleached Kraft Pulp) using conifers and broad-leaved trees, but also GW (Ground Wood), RGP (Refiner Ground wood), PGW (Pressurized Stone Ground wood), CGP (Chemical Ground Pulp), SCP (Semi Chemical Pulp), TMP (Thermo Mechanical Pulp), CTMP (Chemical Thermo Mechanical Pulp) containing more lignin and hemicellulose, and wet paper mainly composed of pulp such as DIP (De-Inked Pulp) essential for recycling of waste paper. Therefore, this effect is further remarkable in not only printing paper such as paper of fine quality, but also newsprint and cardboard base paper (liner and corrugating medium) having a large ratio of DIP and mechanical-type pulp, and white board.
Furthermore, in cylinder drying and heated air drying using the conventional second-kind pressure vessel, moisture diffuses in the sheet material (paper) by a capillary flow and evaporates on the surface of the paper, whereas in super-heated steam drying, moisture evaporates instantaneously in the sheet material and becomes steam, and reaches the surface of the sheet material by a pressure flow. Thus, the moisture evaporation mechanism is quite different between the cylinder drying or heated air drying and super-heated steam drying, and diffusion resistance in the super-heated steam drying is small. As a result, the inner portion of the sheet material becomes porous by several times to several hundred times as measured by means of a pore tester using the mercury press-in method, apparent density of paper decreases by 31%, and moisture absorbing and releasing performance of the sheet material is significantly improved, and thereby the ink absorption property of printing paper and ink jet paper are improved extensively, and the moisture absorbing and releasing properties of stabilized paper and impregnated paper are a also extensively improved.
In a heated gas atmosphere above 130xc2x0 C., the sheet material and the gas-permeable belt having a heat resistant property are brought as close as possible to the blowing ports of the external heating type rotor, putting a walk way outside the hood in view of safety, the internal capacity within the sealed hood can be reduced to one tenth or less compared to the conventional hood, the canvas dryers are eliminated, and a basement area, being a broken paper recovery space, becomes no more unnecessary because the problem of paper break has been settled. As a result, rapid drying is realized, and the plant construction cost can be dramatically reduced.
Moreover, since sealing portions having a short endless heat-resistant sealing belt and a sealing roll, and a sealing pinch roll and a heat-resistant brush seal are respectively provided at the entry portion and the exit portion of the sheet material with respect to the hood interior, outside air can be completely prevented from entering into the hood, the drying efficiency thereby can be remarkably improved. At first, tests were started by making for trial purposes a rotor capable of restraining a wet sheet together with a gas-permeable belt and a wind tunnel provided with caps for blowing super-heated steam and heated air around the rotor. In order to set the internal gas condition to a predetermined values, a gas obtained by heating a circulating gas by an electric heater was blown into the wind tunnel for pre-heating. In the drying test using super-heated steam, in order to introduce the wet sheet into the wind tunnel by means of the fabric belt, various kind of air intercepting apparatus were made for trial purposes and mounted at the entry slit portion to thereby perform the tests. However, a small amount of air entered from narrow slit-like gaps into the inside accompanied by the surface of the sheet and the fabric belt. Hence, it was difficult to realize the oxygen content less than 1% by the oxygen analyzer. Although the oxygen content was considered to be an instrument error, but it was found that the reason was the inflow of a small amount of air. Finally, the air interception method of the present invention makes it possible to reduce the oxygen concentration to 0.0%, and a ortho-test can be started. A zirconia type oxygen analyzer and moisture analyzer applicable to high-temperature super-heated steam were used for measurement of the gas conditions in the wind tunnel, and calibration was always performed using a standard gas before and after the test.
Furthermore, since a gas composed mainly of steam evaporated from the sheet material was reheated to at least 150xc2x0 C., preferably to at least 250xc2x0 C. by a gas circulation unit so as to fill the evaporation chamber with substantially 100% super-heated gas, while controlling the supply and discharge amount of the gas with respect to the hood to thereby set the pressure inside the hood higher than the external pressure (atmospheric pressure), entry of the external air from the outside of the hood through the inlet and outlet of the hood can be completely prevented, which results in improving the drying efficiency.
As a gas permeable fabric belt having a gas permeability of 7500 CCM or higher, (normally, the temperature of the belt becomes considerably lower than the gas temperature), the use of various fabric belts made of PEEK (polyether ether ketone) or PPS (polyphenylene sulfide) or a wire made of metal (made of stainless steel or made of bronze) made it possible to dry the wet sheet material very quickly in a region where the dry shrinkage was restrained. For a type of paper in which the surface smoothness is important, it is desired to sew a thin belt having a good surface quality onto the sheet contacting surface.
Regarding the sealing apparatus proposed in the already registered patent by the present applicant, it was found that the contact portions of the seal pipe and the sealing bracket roll, and the sealing pinch roll became a problem in achieving high speed drying. Therefore, a brush roll method has been developed, to thereby eliminate mechanical contact portions, using a water seal by means of condensed water from the leaking super-heated steam.
Moreover, since the sheet material in a wet and low temperature state has a low surface temperature at the entry portion of the drying apparatus, in order to prevent the paper from sticking to the cylinder and causing frequent breakage of paper, a steam curtain method has been developed by use of a steam box and a suction box. By installing these boxes at the entry portion, problems such as paper break and air leakage were eliminated.
In the present invention, as described above, evaporation and drying in a heated gas atmosphere is made possible by eliminating external air in a specially sealed hood. Therefore, most of the drying process can be completed in a zone where free shrinkage is restrained by the endless gas-permeable fabric belt. That is, the side of the sheet material supported by the rotor has a higher temperature of about 110xc2x0 C. due to high tension by the belt, compared to the conventional temperature of about 100xc2x0 C., and furthermore, this temperature quickly increases with evaporation of moisture, up to a temperature about 25xc2x0 C. less than the heated gas temperature. Then, the moisture contained in the sheet material rapidly moves towards the belt side, and in this state, by blowing heated gas of above 150xc2x0 C. to the surface of the belt at a high speed of more than 50 m/sec., the moisture in the sheet material evaporates directly from the contact surface with the belt, passes through the voids in the belt, and is sucked by a blower through a nozzle-shaped or slit-shaped gas discharge port installed close to the belt. Therefore, unlike to the conventional case, wherein the moist steam in the belt is cooled by the surface of the sheet material at 100xc2x0 C. or lower, and condensed in the belt to become a wet condition, and then returns again to the surface of the sheet material, to re-humidify the surface the sheet material does not occur.
In addition, differing from the conventional construction in which heating is performed from the cylinder side, the present invention has a construction in which heating is performed from the belt side (surface side). Hence, the problem does not occur as before, where moist steam evaporated from the sheet material is accumulated in the belt thereby interrupting evaporation from the surface of the sheet material.
As described above, inside of the belt does not become a wet condition due to the condensed moisture, as before, and hence a fabric belt dryer (a dryer for drying the belt), which has heretofore been required for drying the wet belt, becomes unnecessary. As a result, since a space for installing the dryer for the belt is no more unnecessary, the building cost can be considerably reducted.
In the present invention, since a large amount of heated gas is circulated to maintain the temperature inside the hood always at least 130xc2x0 C., and at the hood entry portion where the sheet material in the wet and low temperature state is introduced into the hood, super-heated steam is made to pass through the cross-section of the sheet by means of the steam box and the suction box, the sheet temperature is thereby raised abruptly by the condensation heat transfer. Accordingly, the dew condensation problem in the hood can be eliminated.
Moreover, a lot of problems associated with the cylinder of the conventional second-kind pressure vessel which has had to be heated with steam from the inside can be solved by the impingement heating of the present invention in that the cylinder is heated by external impingement of a high-temperature gaseous body. The present invention thereby makes it possible to form the cylinder not as a pressure vessel but it is possible to form into a cylindrical shape by combining a plurality of thin metal plates or heat-resistant plastic plates, which enables to divide the cylinder into pieces for transportation and assemble these pieces into a cylinder at a construction site.
The present invention is an external heating method mainly involving heat transfer by means of impingement injection of high temperature heated gas of at least 150xc2x0 C. from the outer periphery of the rotor, and partly a gas heat radiation, and hence, the drying mechanism is fundamentally different from the conventional method. Of course, in the initial stage of drying, the rotor temperature increases by high temperature gas heating, and the sheet material is also indirectly heated from the inside, but in and after the medium period of drying, the sheet temperature increases to higher than the rotor temperature, and heating is effected only by external heating. The rotor (dryer cylinder) which is not heated internally as described above, has until now never known anywhere in the world.
As described above, in the drying process of the present invention, in which different from woven fabric, paper and wet process-nonwoven fabric which are easily broken in wet conditions, and shrink largely in the cross direction due to the continuous tension loaded in the traveling direction through many rolls, and in which interfiber bonding occurs with the evaporation of moisture in the drying process, to thereby cause shrinkage within fibers, a sheet material such as paper or nonwoven fabric having such properties is rapidly dried, while restraining expansion and contraction of the wet sheet material, with one side thereof being brought into contact with a rotatable rotor heated mainly by external heating, and the other side being clamped by an endless gas-permeable belt under high tension, in a sealed hood shut off from the external air, in a pressurized condition slightly higher than the atmospheric pressure, in an atmosphere of heated gas of not lower than 130xc2x0 C., that is, super-heated steam of not lower than 130xc2x0 C., or heated moist air of a dry-bulb temperature of not lower than 130xc2x0 C. and an dew-point temperature of not lower than 65xc2x0 C., or a mixed gas of nitrogen gas of at least 80% containing a small-amount of solvent gas of not lower than 130xc2x0 C. and steam of about 10%. A high-temperature gas in the hood and a gas mainly composed of steam evaporated from the sheet material through the gas-permeable belt are sucked from the gas discharge portion, and circulated and reheated by a gas circulation heating apparatus having a heat source, to be blown at not lower than 150xc2x0 C. from the outer periphery of the rotor, while the remainder is sucked by the gas discharge section and utilized as another heat source. As a result, the sheet material can be dried at high speed of about six times as high as that of the prior art, and by a required heat of about one eighth of the conventional case in a energy saving manner.
In the conventional drying method, the drying rate is slow in the zone where dry shrinkage of the sheet material is restricted by the internal heated cylinder and the fabric belt due to the above-described reasons. On the other hand, in the present invention, drying in an absence of oxygen can be realized by high speed impinging drying at a high temperature, in an atmosphere mainly composed of super-heated steam. At this time, since it is necessary to avoid the risk of burning due to the high temperature, or an oxygen deficiency due to the decrease in oxygen concentration, the walkway, which was conventionally installed inside of the hood, is provided outside the hood in the present invention, to thereby considerably reduce the required space.